PETROLOGY AND DIAGENESIS OF NARRABEEN GROUP SANDSTONES, SYDNEY BASIN,NEW-SOUTH-WALES

The petrology of fluvial sandstones from the Narrabeen Group was inves tigated to document sandstone composition and to assess factors contro lling sandstone diagenesis. The sandstones are dominated by detrital q uartz and lithic grains. The average composition varies in ascending s tratigraphic order from R (rock fragments) > Q (quartz) to Q > R. Majo r diagenetic events in order of occurrence were: (i) formation of hema tite, chlorite and mixed-layer illite/smectite, and first generation o f kaolin (I); (ii) precipitation of calcite, siderite and ankerite; (i ii) dissolution of labile detrital grains and carbonate cement, and fo rmation of second generation of kaolin (II); (iv) quartz overgrowth an d precipitation of illite; and (v) precipitation of calcite and ankeri te. These events were controlled both by detrital mineralogy and pore- water chemistry. Kaolin (I) and chlorite are mutually exclusive. At sh allow burial depth different pore-water chemistry prevailed in differe nt parts of the basin through meteoric water recharge from the southwe st and bacterial activity in mudrocks. In the southwest part of the ba sin repeated meteoric water recharge and aerobic bacterial activity cr eated oxygenated and mildly acidic pore waters and led to the formatio n of kaolin (I). Along the present coastal area and in the far northea st part of the basin a lack of meteoric water recharge combined with c ontinued bacterial activity removed all oxygen and raised the pH value , producing anoxic and neutral to mildly alkaline pore waters and led to the formation of chlorite in litharenites with abundant volcanic ro ck fragments. All authigenic carbonate was formed by direct precipitat ion from pore water. The carbonate type was determined by relative cat ion concentration and Eh condition in the pore water. The amount of Ca -bearing carbonate is related to the detrital mineralogy of the sandst one, with high abundance of volcanic rock fragments favouring formatio n of large amounts of Ca-bearing carbonates. Later dissolution of auth igenic carbonate and labile detrital grains was largely caused by the migration of escaping compactional water containing CO2 which was prob ably sourced from the maturation of organic matter in mudrocks. Kaolin (II) was formed by precipitation from pore waters and replacement of volcanic lithic, feldspar and mica grains as a result of reaction of t hese grains with migrating pore waters. Authigenic quartz overgrowth d eveloped by overlap and/or emergence of initial micro-quartz crystal o utgrowths. Precipitation of illite resulted from the build-up of K+ in the pore waters.